Unproven, perhaps, but to date the so-called 'HIV morning after pill' has been the only treatment available for people who thought they may have been exposed to the virus.

Despite the lack of acceptable data, doctors have assumed that if anti-HIV drugs were effective at keeping long-term positive people healthy, a short-course of treatment over one or two months would also stop the virus embedding itself in someone who has been sexually exposed.

Post-exposure prophylaxis - or PEP for short - has been the 'best guess' treatment based on many years of successful experience of preventing the spread of the virus to health care professionals accidentally injured by contaminated needles.

In both cases though, the key was to prescribe the treatment within 48-72 hours of exposure. The two-to-three-day treatment window was always seen as crucial, tackling the virus before an affected person's immune system had a chance to respond. This is vital because HIV needs the immune system to reproduce itself.

And it now appears the doctors have been guessing right all along. A new study in Brazil has found that the HIV morning-after pill does indeed work.

Dr Mauro Schechter from Universidado Federal de Rio de Janeiro, and colleagues, distributed 'treatment starter packs' to 200 gay men attending sexual health clinics, who were told to self-medicate if they suspected they'd been infected. Overall, the study found that anti-HIV PEPs could cut infection rates by up to 70 per cent.

At the moment, anti-HIV treatment is normally a combination of three or four drugs (hence 'combination therapy') and although they won't eliminate the virus completely - their aim is virtual viral obliteration - most patients seem to be able to cope with what virus is left behind, as long as they keep with the programme.

The upside of this has been the millions of infected people in good health leading productive lives and the closure of Aids wards throughout the developed world, a far cry from just 10 years ago before these drugs were available when HIV was a near certain death sentence.

However, anti-HIV treatment is not risk-free, even for those taking the drugs for a short time. A very small number of patients do react very badly - some near fatally - to some of the drugs involved.

Also there are side effects, ranging from minor skin rashes and nausea to full-blown liver failure and chronic diarrhoea. But, if the treatment is interrupted, levels of HIV will shoot back up, leaving patients prey to highly dangerous conditions such as virulent forms of pneumonia and skin cancers. Positive people on anti-HIV treatment are on it for life.

In the longer term, most pharmaceutical combinations eventually stop working as the ever-mutating virus learns to deal with, and becomes resistant to, a particular drug. This becomes more likely when patients don't take their drugs properly or miss doses. This can be very bad news because doctors naturally prescribe drugs with lesser side-effects first. And if resistance is acquired to several classes of drugs, eventually an infected person will have no treatment options left. Study after study has shown that patients who do best are those who take all their drugs.

The last ten years have seen incredible advances in HIV treatment and most doctors expect this trend to continue, leading to simpler drug regimes with lesser side effects. In May, the availability of a new once-a-day treatment, which combined three drugs, was announced - a massive simplification compared to just a few years ago when some long-term positive people were taking 15-20 pills a day. No one can say for sure what the long-term effects of a protracted anti-HIV drug regime will be. Already, increased rates of cancers and dementia among long-term positive patients have been noted, with some drug combinations leading to increased risk of coronary heart disease.

And out of the shadow of the successful pharmacological fight against HIV has emerged increased rates of other sexually transmitted infections suggesting that the very success of the drugs may be encouraging sexually active people to take more risks, since they no longer see HIV as certain death.

Meanwhile, a vaccine is still a long way off. Dr Mike Youle of London's Royal Free Hospital believes that protection against HIV might one day involve tackling the virus as we now approach malaria. That disease is avoided by a course of treatment taken before, during and after visiting a malaria-prone region plus the use of spray-on insect repellent. For HIV, this might mean a vaccine, (which, like the flu vaccine, would only be effective in 50 per cent of cases), a pre-exposure prophylaxis taken prior to sexual contact and the use of condoms.

For people already infected however, a cure may never come about. But the fact that HIV can now be treated as a chronic condition gives cause for hope.